Venturi multiphase flow measurement based active slug control

Riser slug flow poses a significant challenge to offshore oil production systems, most especially for oil fields in their later life. Active control of slugging through choking has been proven a practical approach in eliminating riser slug flow in oil production pipeline-riser systems. However, existing conventional active slug control systems may reduce oil production significantly due to excessive over choking. Again, some of the existing active slug flow control systems rely on seabed measurements, which are difficult to maintain, costly to install, unreliable, and seldom readily available. This study is an experimental investigation of the feasibility of active riser slug control by taking topside differential pressure measurement from the inlet of the venturi flow meter to the throat. Experimental results indicate that under active slug flow control, the system was able to eliminate slug flow at a higher valve opening when compared to manual choking. A valve opening of 24% with riser base pressure at 2.85 bar from open loop unstable of 23% was recorded, which is superior to manual choking which maintained flow stability up to 21% valve opening with riser base pressure of 3.8 bar

Multiphase severe slug flow control

Severe slug flow is one of the most undesired multiphase flow regimes, due to the associated instability, which imposes major challenges to flow assurance in the oil and gas industry. This thesis presents a comprehensive analysis of the systematic approach to achieving stability and maximum production from an unstable riser-pipeline system. The development of a plant-wide model which comprises an improved simplified riser model (ISRM) required for severe slug controller design and control performance analysis is achieved. The ability of the ISRM to predict nonlinear stability of the unstable riser-pipeline is investi¬gated using an industrial riser and a 4 inch laboratory riser system. Its predic¬tion of the nonlinear stability showed close agreement with experimental and simulation results. Through controllability analysis of the unstable riser-pipeline system, which is focused on achieving the core operational targets of the riser-pipeline produc¬tion system, the maximum stable valve opening achievable with each controlled variable considered is predicted and confirmed through the simulation results. The potential to increase oil production through feedback control is presented by analysing the pressure production relationship using a pressure dependent dimensionless variable known as Production Gain Index (PGI). The performance analyses of three active slug controllers are presented to show that the ability of a slug controller to achieve closed loop stability at large valve opening can be assessed by the analysis of the H∞ norm of the comple¬mentary sensitivity function of the closed loop system, T(s) ∞. A slug controller which achieves the lowest value of the T(s) ∞, will achieve closed loop stability at a larger valve opening. Finally, the development of a new improved relay auto-tuned slug controller algorithm based on a perturbed first-order-plus dead-time (FOPDT) model of the riser system is achieved. Its performance showed that it has the ability to stabilise the riser system at a valve opening that is larger than that achieved with the original (conventional) algorithm with about 4% increase in production

MODELING OF FLOW INSTABILITY IN DEEPWATER FLOWLINES AND RISERS: A CASE STUDY OF SUBSEA OIL PRODUCTION FROM CHINGUETTI FIELD, MAURITANIA

Chinguetti a deepwater oil field development offshore Mauritania is experiencing a rapid decline in its production that resulted to severe flow instability or slugging in flowlines and risers of its subsea oil production system. Slugging initiates oscillations and puts field operator in a demanding situation to manage and control flow instability. It is crucial to have a model to describe flow instability issues in live field conditions. Apparently, there is no applicable model to represent flow instability in deepwater operations. Current available data that represents flow instability in flowlines and risers in live field conditions has not been published in any literature. The available data is mostly from laboratory controlled conditions or laboratory scale ideal condition. Model using laboratory conditions has limited capability that cannot be used to assess severity of slugging. A study was undertaken in which integrated production system of the Chinguetti wells, flowlines and risers were developed using the OLGA transient multi phase flow simulator. Field validation was performed by tuning the models to match field pressures and phase flowrates and instability in the systems. The impact of various changes in operating conditions on the flow instability was examined by simulating the models that included changes in well routings, gas lift injection rates and location of injection points, riser and wellhead choke openings. The severity of flow instabilities for the different operating conditions was categorized by the degree of fluctuations in liquid arrival rates and the characteristics of its liquid slugs, length and frequency. Results from field implementation of the recommended changes in operating conditions indicated improvement in flow stability and oil recovery. From the study, a methodology has been developed to assess the severity of slugging and strategies to mitigate flow stability and productivity in the flowlines and risers ofChinguetti oil production system

The Study of Flow Characteristic in the Riser Pipe from the Subsea Wellhead to Kikeh FPSO

The study focuses on the flow characteristic within the riser pipe in the deepwater environment. Deepwater environment presents significant flow assurance difficulties such as solid formation like hydrate formation that could blocks the fluid delivery to the riser top due to high pressure and low temperature at the sea bed. Furthermore, severe slugging could causes large pressure fluctuation at the riser base and riser top which reduces the production rate and damages the topsides equipments. Hence, extra expenditure needed to be spent to overcome these flow assurance problems. Kikeh Field is the country's first deepwater development in offshore with 1300m of water depth which is operated by Murphy Sabah Oil Co. Ltd and PETRONAS Carigali Sdn. Bhd. One of the wells of Kikeh field, Kikeh-1 well that has 205.5 'F ~f reservoir temperature and 4595 psig of reservoir pressure above the bubble point is adopted to be the case study. The objective of this study is to study the flow characteristic of the oil in horizontal and vertical flow affected by various gas flow rates, oil flow rates and the internal pipeline diameters for Kikeh-1 well. The tendency of hydrate formation and severe slugging formation were also included in the study. PIPESIM software was used to simulate the tendency of hydrate formation based on the Kikeh- 1 well's compositional components, whilst the flow patterns are identified by using Aziz and Mandhane correlations. Different flow patterns and the conditions where hydrate would form contribute a significant reference to deepwater riser and flowline design. The study concluded that at production rate or 13,000 bpd, the superficial gas velocity has to be at below 3 ft/s for horizontal flow and below 0.1 ft/s of modified superficial gas velocity for vertical flow in order to avoid slug flow. Besides, the pipeline should be able to withstand 5,000 psia and operates at least about 750 'F. Such conditions would prevent the hydrate formation to occur in the pipeline. Hence, by understanding the tendency of hydrate formation and the conditions of slug flow to occur which might causes severe slugging, a reliable pipeline could be designed with optimum specifications in order to prevent flow assurance problems

Stabilising slug flow at large valve opening using an intermittent absorber

Slugging is one of the challenges usually encountered in multiphase transportation of oil and gas. It is an intermittent flow of liquid and gas which manifests in pressure and flow fluctuations capable of causing upset in topside process facilities. It can also induce structural defects in pipeline-riser system. The threat of slugging to oil and gas facilities has been known since the early 1970s. This study investigated a new method for slug flow stability analysis and proposed the use of active feedback control and intermittent absorber (a passive device) for hydrodynamic and severe slugging attenuation. The geometry impact on the hydrodynamic slug flow in pipeline-riser systems was established using modelling (LedaFlow and OLGA) and experimental studies. The unit cell model in both software packages, the slug tracking model of OLGA and slug capturing model of LedaFlow were employed for hydrodynamic slug modelling. Three distinct slug regions were reported for a typical pipeline-riser system. The H-region typifies the slug flow regime in the pipeline-riser system due to slug formed in the horizontal pipeline upstream the riser pipe. The V-region represents the slug flow regime due to the riser pipe while the I-region describes slug flow regime where both horizontal and vertical pipes contributes to the dynamics of the slug flow in pipeline-riser system. A simple but yet robust methodology that can be used for pipeline-riser system and slug controller design was proposed. The active feedback control was shown to help stabilise hydrodynamic slug flow at larger valve opening compared with manual valve choking. For the case study, a benefit of up to 5% reduction in riserbase pressure was recorded for the proposed method. This in practical sense means increase in oil production. The analysis also showed that the new slug attenuation device (intermittent absorber) possesses the potential for slug attenuation. Experimental studies showed that the device was able to reduce the magnitude of riserbase pressure fluctuation due to hydrodynamic slugging up to 22%. The absorber enables larger valve opening for both hydrodynamic and severe slugging stabilisation. For severe slugging attenuation for example, a benefit of 9% reduction in riser base pressure was recorded for the case studied. This is of great benefit to the oil and gas industry since this translates to increased oil production. Slug attenuation index (SAI) and pressure benefit index (PBI), have been proposed to quantify the slug attenuation potential and the production benefits of the intermittent absorber respectively. The SAI and the PBI provided consistent results and methods for estimating the slug attenuation potential of the intermittent absorber concept. They could also be used to quantify the slug attenuation benefits of other slug mitigation techniques

Multiphase flow instability and active slug control solutions.

Slugging as a flow assurance challenge is an upsetting condition to the oil and gas industry due to the instabilities it poses on the system. The negative repercussions associated with slug flow stems from the inlet through to the topside facilities where processing is done. Active control has been established as one of the best techniques to eradicate slug and its accompanying challenges however the controller robustness and some setbacks make improvement a necessity. In that vein, the Inferential slug controller which uses a combination of topside measurement signals to produce a single control variable which is more sensitive to slug variations hence can effectively be used to control slug, was invented. Again the robustness of this controller has been in question. This study presents a comprehensive and systematic analysis of the Inferential slug controller design for system stability analysis and maximising throughput from unstable riser pipeline system configurations in the quest to advance this technology. The inferential slug controller’s robustness was assessed by implementing this technique on several pipeline riser systems including U-shape and S-shape riser configurations. Prior to that, the flow behaviour for a wide range of flow conditions was investigated, highlighting the impact of geometry on unstable slug flow through the OLGA flow simulator (modelling) and experiments. New and unused measurement signals from the topside of either the riser/platforms were deployed in the inferential slug control technology to make the controller more sensitive and robust. A simplistic nevertheless robust procedure for designing the inferential slug controller was proposed. Unstable slug flow conditions were observed to stabilise at a relatively larger valve opening compared with that seen in open loop. The inferential slug controller technology is further extended to deal with systems with variable time delay using a proposed modified Smith predictor model. The modified Smith predictor was recorded to improve and stabilise a pipeline riser system which has deteriorated in control performance due to time delay in the system, a resultant of large stroke time in the valve. This in practicality means an increased production through the system. In advancing the ISC technology to be deployed on offshore fields in conjunction with other passive slug mitigation techniques, the slug mitigation potential of pseudo spiral tube (PST) was assessed when installed at the topside of the riser system. The analysis showed that the PST pipe section (spiral and wavy piece) when installed at the topside of the riser system, possesses some mitigation potential. Four different slug regions was identified for the entire pipeline system. The first region being a slug flow occurrence in the system with and without the PST whiles the second region is the region where slugging occurs in the system but disappears when coupled with the PST and the opposite describes the third region. Lastly, the fourth region is described as that region where slugging flow exist for the system coupled with the spiral pipe section and without any PST (plain) but slugging flow disappears when the system is coupled with the wavy pipe section. The wavy or spiral pipe section coupled with the S-shape riser system have slug mitigation capabilities when they are installed at the top of the riser although its effectiveness of slug mitigation depends on the flow condition. This is evident in the significant reduction in the riserbase pressure oscillation magnitude and the significant reduction in the slug envelope (region) when the system was coupled with the wavy or spiral pipe section relative to the plain system.PhD in Energy and Powe

Studies of two-phase intermittent flow in pipelines.

Imperial Users onl

Slug flow regime in a flowline with U-shape riser

A suitable initial point for understanding multiphase flows is a phenomenological description of the mechanism of geometric distributions or flow patterns that are observed. The challenge however is the prediction of the flow patterns for a combination of flow operating conditions and the characteristics of the phases as well as points of transition from one pattern to the other. Different flow patterns occur in different pipeline configurations for which U-shape risers are part. In the quest to stabilise unstable slug flow in the U-shape riser, an experimental study of gas-liquid flow mixture is conducted to understand the behaviour of the flow in the riser. This paper seeks to understand the flow dynamics in a 2-inch internal diameter U-shape pipeline riser system with much emphasis on unstable slug flow. The initiation of this flow instabilities in the U-shape pipeline riser system and the impact of the downcomer on the flow behaviour is investigated experimentally. Understanding the flow behaviour in the U-shape riser could help in developing effective control techniques to stabilise the multiphase flows in the flowlines. Experimentally, flow patterns observed from the U-shape pipeline riser configuration is used to develop a flow regime map which was then compared to that observed in literature and similarly to a purely vertical riser with similar pipe diameter. Thus, a slug envelope was developed for the U-shape riser to help identify which regions slugging could occur in the system

Injectable venturi for slug control.

Severe slugging is a cyclic flow regime which causes intermittent delivery of oil and gas, which could lead to flow separator flooding, production reduction, platform trips and plant shutdown. The large and rapid variation in flow reduces the average flow output, which could be as large as 50 %. This relative inefficiency results in substantial profit losses. This study presents novel methods for severe slugging mitigation. It describes the use of a Venturi and an injectable Venturi for the improvement of system stability, increase in production and hydrocarbon recovery. An injectable Venturi is a Venturi tube that has an opening at its throat, and a pipe inclined at 45° is inserted into this opening. Thus, gas is injected counter to the flow coming from upstream of the injectable Venturi to choke the working fluid passing through the throat of the injectable Venturi. Flow regimes maps, stability maps, stability curves, severe slug envelopes and Hopf bifurcation maps were generated and used to demonstrate the performance of the Venturi and the injectable Venturi in mitigating severe slugging in a pipeline-riser system. The results from the experiment show that with the Venturi or injectable Venturi coupled to the pipeline-riser system, severe slugging was mitigated, the severity of severe slugging was reduced, the operating region of severe slugging was reduced, and stability was achieved at a larger valve opening and lower riser base pressure. Practically, these results imply an improvement to the stability of the system and increase in oil and gas production. Also, these results indicate that oil and gas production can proceed more smoothly, thus, enhancing flow assurance. Potentially, these results will help to extend the operational life of a reservoir further, thus enhancing oil recovery, safe and continuous production of low-pressure wells.PhD in Energy and Powe

Impact of surface choking on gas-lift stability and flow behaviour in oil producing wells

Acknowledgements The authors wish to thank the financial support provided by the University of Aberdeen scholarship awarding section for sponsoring this project.Peer reviewe